For persistant sessions cookies get checked on every request.
They've been updated on every check, but this seems to cause unintended
cookie invalidation due to race condition on concurrent requests.

Some comments in Trac core session update code made me think about this.
After adding a time stap to trac_auth_session cookie and limiting the
update rate to once per hour dropped persistant sessions seem gone.

On a heavily used server this might even correspond to significatly reduced
request processing overhead. Only I had to adapt the authentication cookie
(hash) refresh rate, because it'll get triggered only once per hour now too
instead of twice per request, in practice even less.

Participating Trac environments will accept and trust cookie data offered by
another Trac environment as related to an authenticated sessions.
Option 'auth_cookie_path' in section [trac] must be set to the same value
in all participating Trac environments simultaneously.

OTOH Trac environments with equal 'auth_cookie_path' value on the same host
must share their trac_auth cookie or sessions would get terminated
prematurely when moving between them.